With rapid advance in the field of magnetic recording, further improvements in magnetic recording media are demanded than ever before. High reproduction output is required in short-wave recording, especially in a VTR magnetic recording medium. In view of this, finely dividing of magnetic particles is used to improve the S/N ratio and to make the surface of the magnetic layer smoother. However, when the magnetic powder consists of such fine particles, the coating viscosity increases during the manufacture thereof. This results in an additional problem of a degraded dispersibility of the magnetic powder.
The problem involved in an increase in the viscosity of the magnetic coating is not limited to the degradation in the dispersibility of the magnetic powder. For example, in the coating transfer step during the manufacture of the magnetic recording medium, the transfer of the coating cannot be smoothly performed. This tends to cause local retention of the coating in the transport path and to result in hardening of the coating. This may also lead to drop-out when the coating is used to form a magnetic layer. When such a coating is coated, the coating cannot be easily smoothed after being transferred onto a nonmagnetic support. As a result, a good surface property cannot be obtained. Furthermore, an increase in the viscosity of the coating provides a disadvantageous condition for a thin layer coating which has currently been developed. Thus, there are a wide range of problems which may be encountered with an increase in the viscosity of the coating in the process from the manufacture of magnetic coating to the formation of magnetic layer.
When the magnetic powder is finely divided, i.e., when the specific surface area of the powder increases, the contact interface between the magnetic powder and a coating or a binder increases. This means that an increased energy is required for dispersing the magnetic powder. This problem can be partially resolved by improving a mixing/dispersing unit. However, the degree of such an improvement is limited. In view of this, the conventional method has used a surfactant which also improves the dispersibility. However, with this method, when the amount of the surfactant added is increased for improving the dispersibility, the properties of the layer tend to be impaired. For this reason, this method has not been ideal when high-density recording and high durability which are required for current magnetic recording media, are consiered.